L0301P9 - Fuel Molecules Part 3
__TOC__ Digestion *breaking down for energy in gastrointestinal tract (GIT) Mobilisation *using stored energy Carbohydrates Starch *taken up in our diet *digested before it is absorbed (as glucose) *broken down by amylase *two forms: **amylose ***α1 —> 4 linkages ***linear, unbranched **amylopectin ***α1 —> 4 linkages in chains ***branched at α1 —> 6 linkages Glycogen *stored in liver and muscle *similar structure to amylopectin but more branched **α1 —> 4 linkages in chains **branched at α1 —> 6 linkages Digestion *starts in the mouth **saliva contains digestive enzymes *moves down through the GIT *most digestion occurs in the small intestines *glucose released into the bloodstream then to cells where it is: **immediately used for energy production via glycolysis **stored as glycogen Mobilisation of Stored Carbohydrates *when energy is required i.e. the blood glucose level drops *glycogen is mobilised and converted to glucose 6-phosphate (G6P) **in the liver - G6P converted to glucose and released into the blood to be taken up in tissues **in skeletal muscle, G6P is metabolised to obtain energy during activity ***does not have enzyme to convert G6P to glucose Lipids Digestion *lipids are hydrophobic, thus process for digestion is very different Process #dietary fats form large lipids droplets in GIT #bile salts in the intestinal lumen break droplets into micelles where hydrophobic enzymes can bind with lipids #triglycerides are broken down into fatty acids and monoglycerides and diffuse into cells #*enzyme: pancreatic lipase #re-synthesised into triacylglycerols in the endoplasmic reticulum #triglycerides are packaged with cholesterol and phospholipids in protein-coated chylomicrons (lipoprotein) #chylomicrons enclosed in vesicles enter the lymphatic system by exocytosis Mobilisation of Stored Lipids *adipocytes stores lipid as triglycerides **highly hydrophobic thus much better storage than glycogen **very small cytoplasm, mainly lipid droplet *enzyme triacylglycerol lipase breaks triglycerides down into **glycerol ***converted into acetyl coA ***enter glycolytic pathway **fatty acids ***bind to serum albumin (which is synthesised in the liver) in the blood ***move to specific cells and diffuse in ***undergoes β-oxidation to produce ATP *process is regulated by hormone triggers **adrenaline **glucagon Contrasting Energy Yields *Fatty Acid (16 Carbon) **highest yield **131 ATP produced *Glucose (aerobic) **32 ATP *Glucose (anaerobic) **lowest yield **2ATP Cholesterol Metabolism and Excretion *is a lipid but not a energy source *cannot be digested or metabolised in mammalian cells **due to lack of the appropriates enzymes *removal via GIT through faeces or ~50% converted to bile acids *there is a balance between losses through faecal excretion, absorption and de novo synthesis Feedback Systems *ensure homeostasis *saves resources Negative Feedback *inhibits variable by inhibiting enzymes *e.g. concentration of insulin in the blood Positive Feedback *stimulates variable *e.g. salivation Energy Producing Processes *highly regulated *any factor which disturbs ADP/ATP or NADH/NAD+ ratios will affect balance **may lead to disruption of cellular respiration Alcohol Metabolism *small but significant source of energy in Western adult diet *4-6% ingested kjoules are derived from ethanol in alcoholic beverages *alcoholics may consume >50% of their daily energy as ethanol *ethanol is an convenient energy source **higher energy value than carbohydrates and proteins **and thus it is preferentially metabolised *however can have many adverse effects Absorption *absorbed by all parts of the GIT *diffuses rapidly through cell membranes *rate of absorption differed greatly between people but mainly depends on rate of stomach emptying *food (fats) slows rate of stomach emptying *elimination of alcohol from the body differs greatly between people **depending on gender, race, age, food, biological rhythms, exercise etc Metabolism *ethanol —> acetaldehyde —> acetate *occurs in the liver *produces NADH which affects NAD+/NADH ratio **increased NADH ***prevents citric acid cycle ***pushes production of lactate - lactic acidosis ***pushes production of fatty acid = fatty liver -> alcoholic hepatitis -> cirrhosis Microsomal Ethanol Oxidising System *MEOS is an alternate pathway of ethanol metabolism *occurs in the microsome in the oxidation of ethanol to acetaldehyde *only a minor role in average individuals, but increases after chronic alcohol consumption *acetaldehyde is produced = higher concentrations in the body which has many toxic effects Damage Caused by Alcohol *cirrhosis of the liver *lactic acidosis *hypoglycaemia (combined with bad diet) **citric acid cycle decreased **no oxaloacetate to make glucose *acetaldehyde increase **highly toxic **impairs intestinal absorption of some amino acids, vitamins and minerals **affects liver, brain, heart